Semiconductor polymer compositions comprising a grafted block copolymer of synthetic rubber and polyolefin and carbon black

ABSTRACT

Semiconductive polymer compositions, useful as electrostatic shielding, comprise an electrically conductive filler, such as carbon black, in a polymer matrix comprising a polyolefin and a synthetic rubber.

United States Patent Hartman 1 1 Jan. 21, 1975 SEMICONDUCTOR POLYMERCOMPOSITIONS COMPRISING A GRAFTED BLOCK COPOLYMER OF SYNTHETIC RUBBERAND POLYOLEFIN AND CARBON BLACK [75] Inventor: Paul F. Hartman, Wayne,NJ.

[73] Assignee: Allied Chemical Corporation, New

York, N.Y.

22 Filed: Dec. 20, 1972 21 Appl. No.: 317,020

Related U.S. Application Data [63] Continuation of Ser. No. 34,558, May4, 1970, abandoned, which is a continuation-in-part of Ser. No. 780,165,Nov. 29, 1968, abandoned, which is a continuation-in-part of Ser. No.690,758, Dec. 15,

1967, abandoned. [52] U.S. Cl 252/511, 260/41, 260/41.5, 260/878 [51]Int. Cl..... H0lb l/06, C08c 11/18, C08f 15/04 [58] Field of Search.....252/510, 511; 260/41, 41.5, 260/878 B Primary Examiner-Harvey E. BehrendAssistant Examiner-R. E. Schafer Attorney, Agent, or FirmMichael S..larosz [57] ABSTRACT Semiconductive polymer compositions, useful asclcctrostatic shielding, comprise an electrically conductive filler,such as carbon black, in a polymer matrix com prising a polyolefin and asynthetic rubber.

9 Claims, No Drawings SEMICONDUCTOR POLYMER COMPOSITIONS COMPRISING AGRAFTED BLOCK COPOLYMER OF SYNTHETIC RUBBER AND POLYOLEFIN AND CARBONBLACK This application is a continuation of copending application SerNo. 34,558, filed May 4, 1970, which application is acontinuation-in-part application of application Ser. No. 780,165 filedNov. 29, 1968, abandoned, which application in turn is acontinuation-in-part of application Ser. No. 690,758, filed Dec. 15,1967, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to electricallysemiconductive polymer compositions comprising an electricallyconductive filler material, such as electrically conductive carbonblack, in a polymer matrix comprising a polyolefin and a syntheticrubber.

Organic polymers are normally electrical insulators. For certainapplications, however, such as to provide electrostatic shielding ofpower cables and other articles, it is desirable to utilize a polymercomposition which is electrically semiconductive. It is known thatcertain polymers can be rendered semiconductive by incorporating thereina sufficient amount of an electrically conductive filler, such aselectrically conductive carbon black. The two types of polymers mostgenerally used are synthetic rubbers and polyethylene copolymers;however, even with these polymers more than 30% by weight of carbonblack is normally required to obtain the desired degree of conductivity.(See J. E. Hager, Semiconductive Polyolefin Compounds, Modern Plastics47(1), pp. l42145, Jan. 1970.)

Semiconductive polymer compositions based on olefin polymers andcontaining in excess of 30% by weight carbon black are commerciallyavailable. These compositions are employed extensively as jacketing forpower cables to provide electrostatic shielding. In such applications,however, the polymer compositions must not only be semiconductive, butmost also have other requisite physical properties, particularly tensileproperties. High loadings of carbon black tend to impair these physicalproperties, however, primarily by causing embrittlement. Other drawbacksof high loadings of carbon black include greater expense and greatersensitivity to moisture. Carbon black is notoriously hygroscopic and, asa result, the more carbon black a polymer composition contains, the moreextensively it must be dried prior to extrusion to avoid surfaceimperfections and voids in the extruded product.

It is an object of this invention to provide improved semiconductivepolymer compositions, particularly such compositions containing reducedamounts of carbon black.

SUMMARY OF THE INVENTION We have discovered that polymer compositionscomprising a grafted block copolymer of a polyolefin and a syntheticrubber can be rendered semiconductive by incorporating therein less thanthe normally required amount of electrically conductive filler. Thecompositions of this invention comprise an electrically conductivefiller material in a polymer matrix comprising a grafted block copolymerof a polyolefin and a synthetic rubber, the electrically conductivefiller being present in an amount of from about 5 to 19%, morepreferably 10 to 19%, by volume based on the total volume of the fillerand the polymer matrix. The polymer matrix preferably comprises fromabout 5 to 60 percent, preferably 20 to 60 percent, by weight of asynthetic rubber or blend of synthetic rubbers, and the balance of thepolymer matrix is preferably a polyolefin or blend of polyolefins.

Suitable synthetic rubbers include neoprene (poly(2-chloro-l,3-butadiene), polyisoprene, polyisobutylene, polybutadiene,copolymers of butadiene and styrene containing to 95% (preferably tobutadiene, copolymers of butadiene and acrylonitrile contain ing 40 to(preferably 60 to 85%) butadiene, ethylene-propylene terpolymers(terpolymers of ethylene, propylene and a diolefin), and butyl rubber,which is produced by the copolymerization of an isoolefin having 4 to 7carbon atoms (such as isobutylene) and a minor portion (generally l8%)of a conjugated diolefin having 4 to 8 carbon atoms (such as isoprene).These synthetic rubbers are all well known and are readily availablecommercially.

As used herein, the term polyolefin" refers to polymers of compoundshaving the general formula CH =CHR wherein R is hydrogen, chlorine,acetoxy, phenyl or alkyl radicals having 1 to 4 carbon atoms, andcopolymers of such compounds with each other and with ethylenicallyunsaturated carboxylic acids and esters thereof having 3 to 6 carbonatoms in the acyloxy group and l to 6 carbon atoms in the ester group.Examples of suitable polyolefins include polyvinylchloride,polyvinylacetate, polystyrene, polyethylene, polypropylene,polybutene-l, poly(ethylene-acrylic acid), poly(ethylene-ethylacrylate), and poly(ethylenemethyl methacrylate). The polyolefincomponent of the polymer matrix preferably comprises predominantlypolyethylene, polypropylene, or polybutene-l. As used herein, the termspolyethylene, polypropylene, and polybutene-l include such polymershaving from 0 to 3% by weight of ethylene, propylene, butene-l,pentene-l, or hexene-l copolymerized therewith. More preferably, thepolyolefin component of the polymer matrix is predominantly, preferablyessentially only, polyethylene.

Particularly good results are obtained using grafted block copolymers ofa synthetic rubber and a polyolefin selected from the group consistingof polyethylene, polypropylene, and polybutene-l. A method for preparingsuch block copolymers is disclosed in copending U.S. application Ser.No. 780,165 filed Nov. 29, 1968, the pertinent subject matter of whichis incorporated herein by reference. Especially preferred are blockcopolymers of polyethylene and a synthetic rubber, with the syntheticrubber preferably being butyl rubber.

Grafted block copolymers of polyolefins and synthetic rubbers areprepared by mixing and heating a polyolefin such as polyethylene and asynthetic rubber such as butyl rubber in the presence of a bifunctionalphenolic compound which acts as a grafting vehicle. The bifunctionalphenolic compounds employed in the invention may be essentially eithermonomeric bifunctional phenols or polymeric bifunctional phenols, ineither case having their functionality in the ortho positions with thepara position substituted with an essentially inert substituent, such asalkyl, alkylaryl or arylalkyl radical of up to about 16 carbon atoms,preferably 4 to 12 carbon atoms. The more preferred phenolic compoundsare the polymeric or so-called condensed bifunctional phenoliccompounds. The ortho functionality of the suitable phenols is usuallyprovided by a hydroxy or halogen substituent, the latter preferablybeing chlorine or bromine. The amount of the bifunctional phenolemployed in the present invention may vary fairly widely between about0.3% to 15% by total weight of the polyethylene and rubber to be graftedthereto, depending largely on the amount of rubber in the reactionmixture. The amount of the phenolic compound preferably is between aboutI to 8% by total weight of the substrate polymers. The grafting reactionitself is generally effected by heating the mixed components, desirablywhile maintaining mixing, to a temperature from about 250F. to 425F. Inthe more preferred embodiments the mixture is at the graftingtemperature for between about to 20 minutes.

Electrically conductive filler material suitable for use in thisinvention includes particulate material having a particle size rangingfrom 100 microns to l millimicron, preferably 10 microns to 10millimicrons, and a volume resistivity of less than 0.01 ohm-cm. Thefiller material is preferably electrically conductive carbon black, butother electrically conductive materials, including finely-divided metalssuch as iron, copper, bronze, silver, etc., can also be used.Electrically conductive carbon black is a well known, commerciallyavailable material. It is usually produced by the decomposition ofacetylene (acetylene black) or by the partial combustion of natural gasor liquid in insulated furnaces (furnace black). Electrically conductivecarbon black suitable for use in this invention has a particle size inthe range 10 to 180 millimicrons.

The proportion of electrically conductive filler present in thecompositions ofthis invention is expressed on a volume basis because thevarious fillers which can be employed have widely varying densities.When carbon black is employed as filler in polymer compositions,however, it is customary to express the proportion of carbon blackpresent in the composition on a weight basis. In the compositions ofthis invention, the proportion of carbon black present is preferablyfrom to 30%, more preferably to 26%, by weight based on the total weightof the carbon black and the polymer matrix. In these ranges, thecompositions of this invention have substantially greater electricalconductivity than compositions based on either the polyolefin orsynthetic rubber component alone and containing the same amount ofcarbon black. This result is surprising because one would normallyexpect the degree of electrical conductivity of the two-polymer systemto be simplyan arithmetic average of the electrical conductivities ofthe one-polymer systems.

In containing less electrically conductive filler for a desired degreeof conductivity, the compositions of this invention offer severaladvantages over other compositions. They are less costly to producebecause the conductive filler is normally more expensive, especially ona volume basis, than the polymers employed. Furthermore, at lower fillerloadings the compositions better retain their physical properties,particularly tensile strength, ultimate elongation, stress crackresistance, impact strength and low temperature brittleness point. Withless filler, the compositions are also easier to process, having higherflow rates with less power consumption when extruded. Additionaladvantages include less moisture pickup, less sensitivity to shearhistory with 7 methods known in the art, such as by simply milling amixture of the carbon black and the polymer composition on aconventional rubber mill.

When filler other than carbon black is employed, it is preferably addedto the synthetic rubber before the synthetic rubber is combined with thepolyolefin component. After the filler is added to the synthetic rubbercomponent (whether or not the filler is carbon black), the resultantmixture is then combined with the poly olefin component in accordancewith conventional methods, such as by milling on a rubber mill.

The compositions of this invention can contain minor amounts of otherthermoplastic resins as well as conventional polymer additives, such asantioxidants and other stabilizers, plasticizers, etc.

The compositions of this invention are useful as electrostatic shieldingof electric power cables and other articles, such as explosives andconduits conveying flammable substances in potentially hazardousenvironments. In the case of power cables, conduits and similararticles, the compositions of this invention can be extruded asjacketing over the article to provide the desired electrostaticshielding.

The following examples further illustrate the invention.

EXAMPLE 1 Samples of a grafted block copolymer of polyethylene and butylrubber were blended in a Brabender Plasti-Corder for various periods oftime with various amounts of an electrically conductive carbon. blackavailable commercially from the Cabot Corporation under the trademarkVulcan XC-72. The copolymer was prepared by heating on a two-roll rubbermill a mixture of parts by weight of the polyethylene, 25 parts byweight of the butyl rubber and about 2.5 parts by weight of abifunctional phenolic resin comprising a normally solid brominatedreaction product of pmethylphenol and formaldehyde, obtainedcommercially under the trademark SP-l055. The mixture was heated at325350F for about 3 minutes, then at 350-400F for about 2 minutes. Thepolyethylene had a density of 0.95, a melt index of 0.4, and containedabout 2.0% butene-l copolymerized therewith. The butyl rubber was acopolymer of about 98% isobutylene and 2% isoprene, had a density of0.92, and was obtained under the trademark Enjay 268. The carbon blackhad an inherent density of about 1.8 and a particle size of about 29millimicrons. After being blended with the carbon black, the sampleswere pressed into plaques from which 2-inch discs were cut forresistivity measurement. The surfaces of the discs were silver paintedand a direct current of one volt was applied to the discs. Theresistivity of each disc was determined using a Wheatstone bridge. Theyield strength, ultimate tensile strength, and ultimate elongation ofcertain samples were determined in accordance with appropriate ASTMtests. The results are tabulated below:

Percent Mixing Volume Yield Ultimate Ultimate Percent Mixing VolumeYield Ultimate Ultimate Carbon Time Resistivity Strength TensileElongation Carbon Time Resistivity Strength Tensile Elongation Black(min.) (Ohm-C (P r ng hw l Black (min.) (ohm-cm) (psi) Strengthtpsi)('71) 15 5 1720 No yield d0. 20 3690 l5 1120 peak 1703 21(1 20 1019.4-21.0 2185 2133 140 5 20 10 43.0 do. 20 40.4 2136 2137 161 26 1013.2 26 5 6.2-7.8 do. 20 19.5 N 111 (1911 do. 10 7.4-8.6 2408 2199 98 ldo. 20 8.8-l0.1 d0. 40 l .0

l0 EXAMPLE 2 EXAMPLE 6 The Procedure of Example 1 W5 followed except theThe procedure of Example 5 was followed except the carbon black employedwas ObiHlIlCCl Ulldfil' tl'lC tradecopolymer was prepared using parts byweight ofthc. mark Vulcan C. The tensile properties of thecompostpolyethylene, 50 parts by weight of the butyl rubber, tions werenot determined. The results are tabulated d 5 parts b i h f h h li f ihi l bfilOWI The results are tabulated below:

Percent Mixing Volume Yield Ultimate Ultimate Carbon Time ResistivityStrength Tensile Elongation Black (min.) (ohm-cm) (psi) Strength(psi)("/t) No yield 10 1260 peak 1457 228 10 9.27 do. 20 94.0 26 10 221 do.20 21.1 No yield l34l lUX peak Percent Mixing Time Volume ResistivityEXAMPLE 7 Carbon Black (min.) (ohm-cm) The procedure of Example 4 wasfollowed except the polyethylene and butyl rubber were present as ablend 15 10 1.04 x I0 instead of a grafted block copolymer. The resultsare 20 lo 297 tabulated below:

Percent Mixing Volume Yield Ultimate Ultimate Carbon Time ResistivityStrength Tensile Elongation Black (min.) (ohm-cm) (psi) Strengthtpsi)('4) EXAMPLE 3 When compared with Example 4 this Example demonstratesthat better results are obtained when the polymer matrix is a graftedblock copolymer rather than the The procedure of Example 2 was followedexcept the 0 corresponding p y blend carbon black employed was obtainedunder the trademark Vulcan SC. The results are tabulated below: EXAMPLE8 The general procedure of Example I was followed Car li dii ti l ack iriiinh fififiilfi except the carbon black was obtained under thetrademark Vulcan SC and the polymer matrix consisted of 13 lg 5 a blendof about 66 parts by weight of the polyethylene and 34 parts by weightof polyisobutylene obtained under the trademark Vistanex. The tensileproperties EXAMPLE 4 of the compositions were not determined. Theresults are tabulated below: The procedure of Example 1 was followedexcept the grafted block copolymer was prepared using 50 parts igfi z'sfi g lffii i r by weight of the polyethylene, 50 parts by weight ofthe butyl rubber and 5 parts by weight of the phenolic is :3 2; graftingvehicle. The results are tabulated below:

Percent Mixing Volume Yield Ultimate Ultimate Carbon Time ResistivityStrength Tensile Elongation Black (min.) (ohm-cm) (psi) Strength(psi)/z) 15 5 92 do. 10 134-159 No yield l908 200 do. 20 188 peak 20 1017.8-19.9 25 10 12.6

EXAMPLE 5 COMPARATIVE EXAMPLE A The procedure of Example 1 was followedexcept the Except for the sample containing no carbon black.polyethylene employed was essentially an ethylene hothe procedure ofExample 1 was followed using only mopolymer having a density of 0.919and a melt index the polyethylene instead of the block copolymer. The of1.0. The results are tabulated below: results are tabulated below:

Percent Mixing Volume Yield Ultimate Ultimate Carbon Time ResistivityStrength Tensile Elongation Black (min.) (ohm-cm) (psi) Strength(psi) ll0 -l.2 X 10' 4038 Tears 20 2O 10 108 4004 Tears 19 The fact that thecompositions of this Example tear when subjected to stress and havelower values for ultimate elongation indicates that these compositionsare not as tough as the compositions of the invention. Accordingly, thecompositions of the Example have lower stress crack resistance, lessimpact strength, and impaired low temperature brittleness point, whichare important properties for compositions of this type.

COMPARATIVE EXAMPLE B The volume resistivities of compositionsconsisting essentially of butyl rubber and various amounts of carbonblack (Vulcan XC-72) are indicated in the table below:

Percent Carbon Black Volume Resistivity (ohm-cm) 20% to about 30% byweight, based on the total weight of the carbon black and the polymermatrix.

2. The composition of claim 1 wherein the synthetic rubber is selectedfrom the group consisting of neoprene, polyisoprene, polyisobutylene,polybutadiene, butyl rubber, copolymers of butadiene and styrenecontaining 50 to butadiene, and ethylene-propylene terpolymers.

3. The composition of claim 2 wherein the synthetic rubber is grafted tothe polyolefin through a bifunctional phenolic material wherein thefunctional groups are hydroxy or halogen substituents in orthopositions.

4. The composition of claim 3 wherein the synthetic rubber is butylrubber.

5. The composition of claim 3 wherein the polyolefin is polyethylene.

6. The composition of claim 5 wherein the synthetic rubber is butylrubber.

7. The composition of claim 6 wherein the butyl rubber is present in thecopolymer in an amount of from 20 to 60 percent by weight.

8. The composition of claim 7 wherein the carbon black is present in anamount of from about 20 to 26% by weight based on the total weight ofthe carbon black and the polymer matrix.

9. An electric power cable jacketed with the composition of claim 1.

2. The composition of claim 1 wherein the synthetic rubber is selectedfrom the group consisting of neoprene, polyisoprene, polyisobutylene,polybutadiene, butyl rubber, copolymers of butadiene and styrenecontaining 50 to 95% butadiene, and ethylene-propylene terpolymers. 3.The composition of claim 2 wherein the synthetic rubber is grafted tothe polyolefin through a bifunctional phenolic material wherein thefunctional groups are hydroxy or halogen substituents in orthopositions.
 4. The composition of claim 3 wherein the synthetic rubber isbutyl rubber.
 5. The composition of claim 3 wherein the polyolefin ispolyethylene.
 6. The composition of claim 5 wherein the synthetic rubberis butyl rubber.
 7. The composition of claim 6 wherein the butyl rubberis present in the copolymer in an amount of from 20 to 60 percent byweight.
 8. The composition of claim 7 wherein the carbon black ispresent in an amount of from about 20 to 26% by weight based on thetotal weight of the carbon black and the polymer matrix.
 9. An electricpower cable jacketed with the composition of claim 1.